Although considerable progress has been made in understanding the genetic causes of acute myeloid leukemia (AML), the mainstay of chemotherapy, cytarabine in combination with anthracyclines, have been in use for more than 40 years. Despite their clinical utility, the drugs suffer from considerable toxicity and the long-term results are disappointing: the five-year survival rate for AML patients is less than 20%. The focus of this work is on developing better therapy for the approximately 50% of acute myeloid leukemia (AML) cases that over express the homeobox transcription factor HOXA9, along with the HOX cofactor MEIS1. HOXA9/MEIS1 deregulation has been shown to be pivotal for leukemogenesis as well as an important independent marker for adverse prognosis. Our goal is to better define mechanisms of HOX deregulation in leukemia in order to identify critical interactions or enzymatic activities that could be targeted therapeutically. AML cases with deregulation of these targets are strongly associated with rearrangement of the mixed lineage leukemia gene MLL, a histone H3 lysine 4 specific methyltransferase (involved in approximately 10% of all AML cases), mutation of nucleophosmin (NPM1), resulting in its re-localization to the cytoplasm (NPMc+)(approximately 35% of AML cases) or overexpression of CDX2, which occurs in as many as 90% of normal karyotype AML cases. The most common MLL rearrangements are balanced translocations that fuse MLL to one of a number of nuclear translocation factors. In other cases, MLL is fused to translocation partners that dimerize the truncated MLL molecule. Finally, partial tandem duplication of MLL (MLL-PTD), as a result of duplication of sequences encoding the amino terminal MLL, also deregulate HOX gene expression. AML cases with NPMc+ almost never have MLL rearrangements, but share similar expression profiles as MLL rearranged cases as well as strong cooperativity with FLT3 mutations suggesting these cases share final common pathways for HOX deregulation. We discovered that MLL is recruited to HOX loci through interaction with the polymerase associated factor complex, PAFc, where it regulates HOX gene expression through its histone H3 lysine 4 methyltransferase activity. In addition, we and others have found that common MLL fusion proteins recruit the histone H3 lysine 79 methyltransferase DOT1L to target loci and that this recruitment, along with wild type MLL, are all required for transformation. The experiments in this proposal will lay the groundwork for therapeutic targeting of the MLL-PAF axis or DOT1L in leukemias with varying mechanisms of transformation.)